A coating composition prepared from a fluororesin has a wide range of uses in coating to form a fluororesin layer on a substrate of an article, which needs corrosion resistance, a non-stick property and heat resistance, such as a bread-baking mold, and a rice cooker. However, since the fluororesin is poor in adhesion to a substrate made of a metal, a ceramic or the like because of the non-stick property thereof, the substrate is coated with a primer having an affinity for both the fluororesin and the substrate in advance.
The fluororesin layer is generally required to be thickened in uses requiring corrosion resistance. In order to thicken the fluororesin layer, it is necessary to repeat coating of applying a powder coating composition comprising a fluororesin and baking the applied powder coating composition at temperatures not lower than the melting point of the fluororesin. The primer is required to have heat-resistant adhesion capable of withstanding the long-duration baking at elevated temperatures and maintaining adhesion to a substrate and the like.
As the primer excellent in heat-resistant adhesion, there has been widely adopted a primer based on chromium phosphate, having excellent resistance to long-duration baking at elevated temperatures, until today. However, since there is growing awareness of environmental issues, a development of a chromium-free primer, which does not contain hexavalent chromium but has a strong heat-resistant adhesion comparable to that of the primer based on chromium phosphate, has been strongly desired over the years.
As the chromium-free primer, a combination of a fluororesin and a various binder resin has been conventionally studied. As the binder resin, there was proposed the use of polyphenylene sulfide (PPS) from the viewpoint of heat resistance. However, PPS had a problem in that PPS was poor in compatibility with the fluororesin and adhesion to the fluororesin was insufficient.
In order to improve the adhesion to the fluororesin, it was proposed that polyamide-imide (PAI) and/or polyimide (PI) are/is added to PPS as the binder resin in the chromium-free primer (see, for example, Patent Document 1), in an example in this publication, PAI and PPS are used in a ratio of 1:15 to 1:20.
As the chromium-free primer using PPS and PAI as the binder resin, one using PAI and PPS in a ratio of 3:1 to 1:3 was also proposed (see, for example, Patent Document 2). However, this chromium-free primer has a feature in blending two kinds of fluororesins differing in melt viscosity each other in a specific ratio in order to provide a water-based primer capable of applying to a smooth surface, and there was a problem in that heat-resistant adhesion was deteriorated due to a long-duration baking.
As the binder resin of the chromium-free primer, one having PAI and PPS in a ratio of 1:1 is known (see, for example, Patent Document 3), but there was a problem in durability for hot water.
As a coating having a improved heat-resistant adhesion, a coating composition has also been proposed (for instance, see Patent Document 4) that comprises a macromolecule compound (A) containing amide group, and an anti-oxidizing material (B), wherein the anti-oxidizing material (B) accounts for 0.1 to 20% by mass of a total amount of the macromolecule compound (A) containing amide group and the anti-oxidizing material (B). Nonetheless, greater heat-resistant adhesion is still required.    Patent Document 1: Japanese Patent Application Publication No. S53-74532    Patent Document 2: U.S. Pat. No. 5,789,083    Patent Document 3: Japanese Patent Application Publication No. H08-322732    Patent Document 4: WO 2004/048489